Geomagnetic reversal dating
We present Cassignol technique K–Ar dating of lava flows from La Palma (Canary Islands, Spain) that bracket the Matuyama–Brunhes transition.An age of 821 ± 13 ka obtained for a transitionally magnetized flow (LS118) provides the first volcanic evidence for a geomagnetic excursion occurring about 40 kyr prior to the transition.Sedimentary rocks can also indicate these directions: when sediment is deposited gently in a low-energy environment such as the deep ocean floor, magnetized grains of magnetite and hematite will orient themselves to the Earth's magnetic field like so many tiny compass needles, indicating the directions of the north and south magnetic poles at the time the sediment was deposited.When geologists realized that this was the case, it was immediately obvious that studying the magnetism of ancient rocks would tell them about the Earth's magnetic field as it existed in the deep past: paleomagnetism.The study of paleomagnetism led to the discovery of magnetic field reversals.At present the Earth's magnetic field exhibits what is known as normal polarity: that is, it has the magnetic north pole near the geographic north pole, and the magnetic south pole near the geographic south pole.Champion 1997) and are clearly associated with field intensity minima (e.g. In fast magnetic acquisition media, such as volcanic flows, they can display a local signature, which is highly site dependent (e.g. An excursion can be characterized as an interval of time during which the dipole field was low, but not necessarily associated with transitional directions at all times during this interval, depending on the geometry of the fast varying non-dipole components (e.g. The purpose of the present study is to precisely date an excursion identified in a volcanic sequence from La Palma (Quidelleur & Valet 1996).
X-ray Fluorescence Microscopy work at the Australian Synchrotron has been used to map the iron distribution in the speleothems and in tandem with the demagnetisation spectra has enabled the mineralogy and mode of acquisition of remanence to be determined and the potential effects of recrystalisation on the palaeomagnetic signal to be accessed.
Identifying and directly dating Plio-Pleistocene geomagnetic reversals and events from speleothems at South African archaeological and fossil bearing palaeocaves: implications for extending archaeomagnetic records1503 GEOMAGNETISM AND PALEOMAGNETISM / Archeomagnetism, 1105 GEOCHRONOLOGY / Quaternary geochronology, 1513 GEOMAGNETISM AND PALEOMAGNETISM / Geomagnetic excursions, 1535 GEOMAGNETISM AND PALEOMAGNETISM / Reversals: process, timescale, magnetostratigraphy In the last 10 years palaeomagnetic research on speleothems from archaeological and fossil bearing palaeokarst in northern South Africa has led to the identification of apparent short geomagnetic field events that were initially thought to represent one or both of the Réunion events.
More recently the development of uranium-lead dating techniques for speleothem in the 5 Ma to 500 ka time range has allowed us to directly date these events for the first time, as well as date more recently discovered events and reversals.
We can see today that magnetic minerals in sedimentary and igneous rocks align themselves with the present direction of the magnetic field; and the physics of this is well-understood — this is just what they ought to do.
We can also see that some rocks laid down in the past are magnetically aligned in the opposite direction.
But studies of the paleomagnetism of ancient rocks showed evidence that in the past the Earth has sometimes had reversed polarity, with the magnetic north pole in the southern hemisphere and vice versa.